This invention relates to a digital optical position sensor system for sensing the position of one or more objects in an electrically noisy environment such as a factory or aircraft, more particularly to a sensor that employs wavelength-division multiplexing for optical transport of signals.
Optical signal transport is advantageous because it confers immunity from electromagnetic interference. A conventional digital optical position sensor employs an optical position encoder in which position is indicated by black and white markings in a set of parallel or concentric tracks on a movable code plate or disk. Each track is scanned by an optical element such as a collimator. The collimator receives light from an optical fiber, directs the light onto the track, and returns the reflected light (if any) to the fiber. The fiber terminates at an optical coupler connected to a light source, which sends light pulses into the fiber, and to a photosensor, which converts the returning light to an electrical signal.
One problem with this conventional sensor system is that a separate fiber, coupler, and light source are required for each track. A twelve-bit sensor, for example, requires twelve fibers, couplers, and light sources. Since a factory or aircraft subsystem may include many different sensors, and since aircraft subsystems in particular must be made redundant for reliability, the quantity of fibers, couplers, and light sources can become quite large, e.g. 360 of each item for a triply redundant system with ten twelve-bit sensors (3.times.10.times.12=360). This is a disadvantage from the standpoint of space and weight as well as for economic reasons.
Another problem found in conventional sensors is that, due to vibration for example, the code plate or disk will not always be aligned at right angles to the optic axes of the collimators. As a result, light reflected from a given track may miss the corresponding collimator.
One solution often adopted to this last problem employs a code plate with a pattern of windows that transmit light and opaque portions that absorb or reflect light. The collimators are located on one side of the code plate, and the photosensors on the other side. This enables the optical signals to be picked up reliably, but has the disadvantage that the photosensors generate electrical signals, which must then be transmitted over conventional wiring instead of through optical fibers, with attendant risk of electromagnetic interference.